Photoelectrostatic members

ABSTRACT

A PHOTOELECTROSTATIC MEMBER (SHEET MATERIALS SUCH AS PAPER) COATED WITH A LAYER OF A PHOTOCONDUCTIVE INSULATING MATERIAL SUCH AS ZINC OXIDE DISPERSED IN A SPECIAL POLYURETHANE RESIN PRODUCED FROM A FATTY ACID ESTER OF A POLYHYDRIC ALCOHOL, WHEREIN THE POLYHYDRIC ALCOHOL PORTION OF THE ESTER CONTAINS AT LEAST TWO FREE HYDROXYL RADICALS, AND AN ORGANIC POLYISOCYANATE.

United States Patent Oflice 3,698,895 Patented Oct. 17, 1972 US. Cl. 961.8 9 Claims ABSTRACT OF THE DISCLOSURE A photoelectrostatic member (sheet materials such as paper) coated with a layer of a photoconductive insulating material such as zinc oxide dispersed in a special polyurethane resin produced from a fatty acid ester of a polyhydric alcohol, wherein the polyhydric alcohol portion of the ester contains at least two free hydroxyl radicals, and an organic polyisocyanate.

CROSS-REFERENCE TO RELATED APPLICATION This application is a continuation-in-part of my copending application Ser. No. 618,352, filed Feb. 24, 1967, now abandoned.

INTRODUCTION This invention relates to improved photoelectrostatic methods, members and processes for the production thereof. More particularly, it relates to photoelectrostatic members in which the photoconductive insulating material is bonded to the conductive base or substrate by a special polyurethane resin, thereby forming a photoconductive insulating layer composed of the photoconductive insulating material and the resin on the conductive base or substrate.

Photoelectrostatic members are useful in the electrostatic copying process wherein a conductive base, such as metal or paper having a photoconductive insulating layer thereon is electrostatically charged in the dark. The charged photoconductive layer is then exposed to a light image. The light causes the photoconductive insulating material, such as zinc oxide or selenium, to become electrically conducting and the charge is dissipated rapidly from the surface of the photoconductive insulating layer to the conductive base or substrate, the rate of charge dissipation being proportional to the intensity of the light to which any given area is exposed. After such exposure, the surface of the photoconductive insulating layer is contacted in the dark with electroscopic particles. These particles adhere to the areas where the electrostatic charge is retained, thus forming a powder image corresponding to the electrostatic image. The powder image can then 7 be transferred to a sheet of transfer material to form a print, or, where the base material is a relatively inexpensive material such as paper, the powder image may be fixed thereto directly to form a print.

In the past the photoconductive insulating material in fine particle form has been dispersed in organic resin binders such as ethyl cellulose, polystyrene, nitrocellulose, vinyl polymers, chlorinated rubber, acrylic resins and similar materials. Silicones have been mentioned as satisfactory binder materials (Middleton et a1. Pat. No. 3,121,006). Reaction products of polyester resins with organic diisocyanates have also been recommended as binding agents for photoconductive insulating materials in photoelectrostatic materials .(Bunge et al. Pat. No. 3,025,160). Polystyrene is described as a binder (Sugarman et al. Pat. No. 2,862,815).

BRIEF DESCRIPTION OF INVENTION In accordance with this invention it has been found that photoelectrostatic materials or members in the form of electrically conductive bases or substrates coated with a thin layer of a photoconductive insulating material in a resin binder which is a combination formed of a polyhydroxy ester with an organic polyisocyanate, have particularly desirable properties which make them of value in photoelectrostatic processes. This resin binder combination has the advantages of providing high light sensitivity (rapid light response rate) to the photoelectrostatic materials with the ability to reproduce high fidelity and high quality tonal images with improved print density. In addition, this resin binder provides photoconductive insulating layers which have desirable dark decay rates and dark voltage retention characteristics.

Light sensitivity is particularly desirable in ofiice photocopying equipment where the time required to make a copy is preferably as short as possible. A photoelectrostatic copying material or member having improved light sensitivity of the order achieved by the present invention can be translated into design features of photoelectrostatic copying machines which have the ability to process photocopies at a faster rate than prior machines and with less intense radiation sources than have been used heretofore. Such machines require less power and produce less heat, making for simpler construction. Improved light sensitivity in the photoelectrostatic copying material also obviates the need for including large quantities of highly colored dyes to boost the sensitivity of the photoelectrostatic member. By eliminating colored dyes, particularly in photoelectrostatic papers, it is possible to produce white or nearly white sheets. Moreover, because the speed of processing a photoelectrostatic copy is dependent on light sensitivity and the saturation charge level of the photoelectrostatic member, it is possible with high light sensitivity materials to produce copies that are properly exposed at high speed with little or no background color.

The light sensitivity of the photoelectrostatic materials prepared in accordance with this invention is at least 1.6 times as great as the sensitivity of photoelectrostatic layers formulated with silicone, silicone-alkyd or uralkyd resin binders. The speed of these photoelectrostatic materials is at least 1.25 times that of similar materials formulated with silicone-alkyd resins and at least 2.0 times that of the' bility toward pre-exposure, and performance in lithographic applications.

The various combinations of polyhydroxy esters with organic polyisocyanates produce a special group of polyurethane resins which are compatible with large quantities of zinc oxide pigment and are more effective than uralkyd resins in photoelectrostatic papers. They are substantially lower in cost than are the silicone resins and have significant commercial advantages over such resins. In addition, the photoelectrostatic members produced with these resins have the ability to reproduce high fidelity and high quality tonal images with improved print density when compared to prior art resin binders. In addition, the photoelectrostatic materials produced with such resin binders have desirable dark decay rates and dark voltage retention.

It is an object of this invention to provide photoelectrostatic members having improved properties, particularly with respect to light sensitivity, speed, saturation voltage, dark voltage retention and image density. It is another object to provide methods for producing such photoelectrostatic members. It is a further object to provide photoelectrostatic materials wherein the photoconductive insulating materials are bonded to the substrate or base with a. novel polyurethane resin binder. These and other objects are apparent from and are achieved in accordance with the following disclosure.

DETAILED DESCRIPTION OF INVENTION The photoelectrostatic members which comprise this invention comprise a base or substrate which is electrically conducting and has a resistivity in the range from to 10 ohm-centimeters and preferably in the range of 10 to 10 ohm-centimeters. The substrate or base can be formed of metals such as aluminum, copper and steel, of metal foils such as aluminum and tin, and of paper, plastic, cloth and other materials having the proper electrical resistivity. When paper, cloth or plastic are used as the base materials, the electrical resistivity thereof may be adjusted by the addition of conductivity-increasing materials such as inorganic salts and by control of humidity.

In the production of photoelectrostatic members or materials, the surface of the substrate or base is treated with a thin film, layer or coating of a suspension of finely divided particles of photoconductive insulating material and the polyurethane resin binder in an organic solvent. Upon evaporation of the solvent, a layer of photoconductive insulating particles in the organic resin binder adheres to the surface of the substrate or base. The resulting product is a photoelectrostatic material or member which can be used in xerographic processes as described above.

The photoconductive insulating material can be any of the well known materials of this type such as vitreous selenium, sulfur, oxides of zinc, aluminum, titanium, lead, antimony, bismuth, cadmium, mercury, molybdenum, and copper and sulfides, selenides and tellurides of these metals. Other inorganic materials such as zinc titanate, arsenic trisulfide, lead chromate and cadmium arsenide can be used. In addition, organic materials such as anthracene and anthraquinone are operative.

The amount of photoconductive insulating material suspended in the polyurethane resin binder can be varied over relatively wide ranges. One part by weight of this resin binder can be combined with as little as one part by Weight of photoconductive insulating material or with as many as 50 parts by weight of photoconductive insulating material. Preferably, 6 to 30 parts of photoconductive insulating material per part of resin binder is preferred.

The resin binder of the present invention is made by interaction of an organic polyisocyanate with an aliphatic monocarboxylic fatty acid ester of a polyhydric alcohol containing at least three hydroxyl radicals, wherein the polyhydric alcohol portion of said ester contains at least two free hydroxyl radicals. The resin binders of this invention have molecular weights ranging from about 7,000 to 30,000 or higher. The polyurethane resins are produced from hydroxy esters by reaction with the organic polyisocyanate, the isocyanate groups reacting with available hydroxyl radicals of the esters to produce urethane linkages.

The hydroxy esters from which the polyurethanes are produced include fatty acid esters of polyols which contain three or more hydroxyl radicals. Suitable acids include those containing 10 to 24 carbon atoms, such as oleic, linoleic, stearic, palmitic, lauric, linolenic and ricinoleic. These acids can be obtained from vegetable oils such as corn, peanut, cottonseed, palm, linseed, soya, safllower, castor and dehydrated castor oils. The polyols include glycerol, trimethylolethane, mannitol, trimethylolpropane, sorbitol and pentaerythritol. They are partially esterified with the fatty acid to produce compounds having two or more free hydroxyl radicals in the ester moieties.

In the production of the partially esterified polyols, the polyhydric alcohol component is used in molar excess so that the proportion of alcoholic hydroxyl groups to carboxyl groups is greater than one. The fatty acids render the resins flexible and control the molecular weight and acid number thereof. Preferred are aliphatic polyethylenically unsaturated monocarboxylic fatty acids which permit the resin once it is applied in a thin layer on a suitable substrate to undergo cross-linking, forming a hardened film.

The hydroxy esters can be produced by esterification of the monocarboxylic acid and the polyol by conventional procedures. Acid chlorides and anhydrides can be used in lieu of the acids. The preferred procedure is transesterification whereby a source of fatty acid such as a vegetable oil is reacted with a polyol in the presence of a transesterification catalyst such as an inorganic acid (e.g., HCl or H 50 or a weak base such as an alkali metal or alkaline earth metal salt of an organic acid (e.g., sodium acetate or calcium acetate). For instance, when a vegetable oil such as cottonseed oil is reacted with a mole of pentaerythritol, the product is a mixture of monoglycerides of palmitic, oleic, linoleic, stearic and other fatty acids present in cottonseed oil and monoand di-esters of pentaerythritol and the same fatty acids. This mixture of hydroxy esters contains (on the average) at least two hydroxyl radicals per molecule.

It is often desirable to include certain monohydric alcohols such as butanol, hexanol or cetyl alcohol in quantities of 1-10% of the weight of the total alcohol (polyol) content of the hydroxy esters for the purpose of modifying and controlling the molecular weight of the final polyurethane resins. The hydroxy esters are produced by known methods from the carboxylic acids and the polyhydric alcohols, such as by thermal esterification in an inert atmosphere and under reduced pressure at elevated temperature.

The hydroxy esters are reacted with organic polyisocyanates to produce polyester-polyurethane resins. Genrally, the amount of polyisocyanate used is 10% to 40% of the weight of the hydroxy ester. The hydroxy ester and the polyisocyanate are heated together, preferably in an inert atmosphere, whereby the isocyanate radicals react with the free hydroxyl radicals to produce urethane linkages. A wide range of organic polyisocyanate materials can be used. The common and preferred materials are tolylene-2,4-diisocyanate and tolylene-2,6-diisocyanate or mixtures thereof. Other suitable diisocyanates include methylene diisocyanate, p-phenylene diisocyanate, 4,4-biphenylene diisocyanate, IS-naphthylene diisocyanate, p,pdiisocyanodiphenylmethane, 2,7-naphthylene diisocyanate, m-xylylene diisocyanate and hexamethylene diisocyanate.

The reaction of the organic polyisocyanates with the polyester resins is conducted at temperatures in the range from 80-200 C. for periods of time varying from minutes to 4 hours.

Vinyl aromatic polymers can comprise 5% to 35% of the total resin binder weight and can be chosen from polymers of styrene, divinylbenzene, vinyltoluene, divinyltoluene, methylstyrene, 2,4-dimethylstyrene, vinylnaphthalene, phenylstyrene, vinylfluorene and related vinylarene polymers, as well as related vinyl aromatic polymers such as those of N-vinylcarbazole, N,N-diphenylacrylamide and vinyldibenzofuran. These resin binders can be made by the interaction of a vinylaryl monomer with a polyhydric alcohol ester having multiple pendant polyethylenically unsaturated fatty acid ester groups as Well as unreacted hydroxyl groups on the polyol. The vinylaryl groups attach to the hydroxy ester through free radical polymerization and partial crosslinking of some of the sites of unsaturation on the vinylaryl groups and fatty acid ester groups leaving unaffected the remaining hydroxyl groups on the polyol moiety. The organic polyisocyanate is then reacted with the remaining free hydroxyl radicals on the ester to form urethane linkages. The resulting resin is a polyvinyla'ryl-polyurethane resin having pendant fatty acid ester groups wherein the polyvinylaryl groups are chemically bonded to some of the sites of ethylenic unsaturation of said fatty acid groups and the urethane linkages are formed on the free hydroxyl radicals of the polyhydric alcohol. The remaining sites of unsaturation on the fatty acid groups will crosslink by air oxidation after application to the substrate. These resin binders preferably have molecular weights ranging from about 10,000 to 20,000.

In the production of the resin coating, the polyurethane resins are dissolved in a suitable solvent, such as naphtha, toluene or xylene or mixture of toluene or xylene with butanol, and the finely divided photoconductive insulating material (e.g., zinc oxide) is admixed with good agitation with the resin solution, preferably in a ball mill or similar mixing and grinding equipment. For example, a polyurethane resin produced from 24.5% tolylene diisocyanate and 75.5% unsaturated polyester resin having a molecular weight of approximately 10,000 and an acid number of less than 10 was dissolved in xylene to produce a coating solution containing 50% resin solids by weight.

From the foregoing description, it is seen that the hydroxy ester components of the polyurethane resins are monomers (or mixtures of monomers) of the following general formula:

wherein .R is the aliphatic radical of a fatty acid of the formula RCOOH, X is a polyol nucleus, n is an integer from 1 to 3, inclusive, and m is an integer from 2 to 3, inelusive. The hydroxy esters are usually mixtures of esters formed by esterification of part of the hydroxyl radicals of the polyols with fatty acids. The esterification is so controlled that at least two free hydroxyl radicals (on the average) remain on the polyols. These hydroxyl radicals subsequently react with the organic polyisocyanate to form polyurethane resins in accordance with known procedures.

The invention is disclosed in further detail by means of the following examples which are provided for purposes of illustration only. It Will be understood by those skilled in the art that various modifications in materials, relative proportions and operating conditions can be made within the disclosure of this application without departing from the invention as herein described.

EXAMPLE 1 A mixture of 53 parts of soya oil and 12.5 parts of technical grade pentaerythritol (about 86% monopentaerythritol and balance polymers) was heated to about 200 C. in a nitrogen atmosphere. Then 0.1 part of calcium acetate was added and the mixture heated to 240 C. with agitation until the acid number of the mixture was less than 10. The mixture was cooled, diluted with 100 parts of toluene and warmed to 75 C. About 20 parts of tolylene diisocyanate (:20 2,4-isomer:2,6-isomer) was added with agitation at a rate such that the temperature of the reaction mixture did not exceed C. After the tolylene diisocyanate was added, the mixture was heated to C. and kept at that temperature for 2 hours. Then the mixture was cooled and 600 parts of toluene was added. The resulting solution was filtered and the filtrate charged to a ball mill with 640 parts of electrophotographic grade zinc oxide (American Zinc Company ZZZ-61) and the mixture was milled until the zinc oxide was well dispersed in the solution of the resin. A solution of 0.03 part of rosin yellow and 0.04 part alphasurin in 15 parts of methanol was added to the pigment-resin-solvent blend along with 45 parts of cobalt naphthenate dryer (6% solution in xylene). Milling was continued until a Hegman grind of 4 to 6 (NS scale) was obtained. The resulting coating mixture was applied to a conductively treated paper, 60 pounds basis weight (25 by 38-500) at a rate of 12 pounds per 3000 square feet dry coverage, and the solvent evaporated. A flexible film formed which crosslinked at room temperature at the unsaturated sites in the fatty acid portion of the polyester.

Electrical properties of the coated paper Saturation voltage 900 Dark decay, in volts per second 6.0 Light decay, in volts per foot candle second 105.1 Speed (f.c.c.)- 0.132 Maximum density image area (photo-volt reading) 1.90 Background density non-image area (photo-volt reading) 0.20

The coated paper described was white in color and could be successfully processed after equilibration in environments of a broad range of relative humidities.

I claim:

1. A photoelectrostatic member comprising the combination of an electrically conductive substrate and a layer of one part of inorganic photoconductive insulating particles dispersed in one to fifty parts of a resin binder containing at least 65% by weight of a polyurethane resin produced from an aliphatic monocarboxylic fatty acid ester of a low molecular weight polyhydric alcohol of the formula (RCOO),,X(OH) wherein R is an aliphatic hydrocarbon radical containing 9 to 23 carbon atoms, inclusive, X is a polyol nucleus selected from the group of polyols consisting of glycerol, trimethylolethane, mannitol, trimethylolpropane, sorbitol and pentaerythritol, n is an integer from 1 to 3, inclusive, m is an integer from 2 to 3, inclusive, and the sum of n and m is equal to the number of hydroxyl radicals of the polyol, and an aromatic diisocyanate wherein the isocyanate radicals are attached to benzene rings.

2. A photoelectrostatic member as defined by claim 1 wherein the electrically conductive substrate has a resistivity in the range from 10 to 10 ohm-centimeters and the photoconductive insulating layer thereon comprises a polyurethane resin binder containing finely divided particles of a. photoconductive insulating material dispersed therein, said polyurethane resin binder comprising the reaction product of a pentaerythritol fatty acid ester and an organic diisocyanate, said pentaerythritol ester having at least two free hydroxyl radicals.

3. A photoelectrostatic member as defined by claim 2 wherein the photoconductive insulating layer comprises one part by weight of polyurethane resin combined with about one part to about 50 parts by weight of photoconductive insulating material.

4. A photoelectrostatic member as defined by claim 3 wherein the electrically conductive substrate is paper and the photoconductive insulating material is zinc oxide.

5. A photoelectrostatic member as defined by claim 4 wherein the electrically conductive substrate is metal and the photoconductive insulating material is selenium.

6. A photoelectrostatic member as defined by claim 1 wherein the resin is a copolymer of a 'vinylarene, a polyethylenically unsaturated fatty acid ester of a polyhydric alcohol, and an organic polyisocyanate.

7. A photoelectrostatic member as defined by claim 6 wlfiarein the vinylarene is styrene.

8. A photoelectrostatic member as defined by claim 6 wherein the vinylarene comprises 5% to 35% of the total resin weight.

9. A photoelectrostatic member as defined by claim 8 wherein the vinylarene is styrene.

References Cited UNITED STATES PATENTS 3/1962 Bunge et a1. 96-1 6/1967 Brack 260-775 9/1968 Stahly 96-15 1/1968 Ehrlich et al. 260-18 10/1968 Mammino 96-15 CHARLES E. VAN HORN, Primary Examiner M. B. WITTENBERG, Assistant Examiner US. Cl. X.R. 

